1. Field of the Invention
The present invention relates generally to the field of oxygenators used to increase the oxygen level in a patient's blood. More particularly, the present invention involves a percutaneous oxygenator that can be positioned within a patient's body (e.g. in the inferior vena cava, superior vena cava, the right atrium of the heart, or any combination thereof) and then repeatedly inflated and deflated to minimize streaming of the blood flow around the oxygenator, and thereby maximize the cross-diffusion of oxygen and carbon dioxide.
2. Statement of the Problem
Many types of blood oxygenators are well known in the art. For example, during open heart surgery, the patient is interconnected with an external oxygenator, commonly known as a heart-lung machine, which introduces oxygen into the blood system. Most types of oxygenators use a gas-permeable membrane. Blood flows along one side of the membrane, and oxygen is supplied to the other side of the membrane. Given a sufficient pressure gradient between the oxygen supply and the blood, the oxygen will diffuse through the membrane and into the blood. In addition, carbon dioxide will tend to diffuse from the blood through the membrane.
In other situations, a smaller, implantable oxygenator may be sufficient to adequately supplement the patient's cardiopulmonary function by marginally increasing the oxygen content of the patient's blood. For example, patients suffering from emphysema, pneumonia, congestive heart failure, or other chronic lung disease often have blood oxygen partial pressures of approximately 40 torr. A relatively small increase of 10% to 20% is generally sufficient to adequately maintain the patient. This is a particularly desirable alternative in that it avoids the need to intubate the patient in such cases. In addition, temporary use of this type of oxygenator is sufficient in many cases to tide the patient over an acute respiratory insult. Placing such patients on a conventional respirator is often the beginning of a progressive downhill spiral by damaging the patient's pulmonary tree and thereby causing greater dependence on the respirator.
The effective rate of diffusion in percutaneous oxygenators can be limited in some instances by the problem of "streaming" or "channeling", in which the blood stream establishes relatively stable patterns of flow around and through the oxygenator. Portions of the oxygenator are exposed to a relatively high velocity, turbulent flow of blood. These conditions tend to increase cross-diffusion of oxygen and carbon dioxide. However, other portions of the oxygenator are exposed to a low velocity, laminar flow of blood which reduces diffusion of gases. Those portions of the oxygenator immediately adjacent to the regions of high blood flow may continue to experience high rates of diffusion, but the remaining portions of the oxygenator tend to have relatively low diffusion rates. Thus, the overall diffusion rate of the oxygenator can be substantially diminished by streaming.
A number of devices and processes have been invented in the past relating to different types of oxygenators, including the following:
______________________________________ Inventor Patent No. Issue Date ______________________________________ Bodell 3,505,686 Apr. 14, 1970 Burton 4,159,720 July 3, 1979 Kopp, et al. 4,346,006 Aug. 24, 1982 Mortensen 4,583,969 Apr. 22, 1986 Taheri 4,631,053 Dec. 23, 1986 Kitagawa, et al. 4,743,250 May 10, 1988 Berry, et al. 4,850,958 July 25, 1989 Hattler 4,911,689 Mar. 27, 1990 Hattler, et al. 4,986,809 Jan. 22, 1991 ______________________________________ Tanishita, et al., "Augmentation of Gas Transfer with Pulsatile Flow in the Coiled Tube Member Oxygenator Design", 26 Trans. Am. Soc. Artif. Intern. Organs 561 (1980).
Bodell demonstrates the general concept of using gas-permeable fibers to boost the oxygen level of blood. FIGS. 6 and 10 show two variations of this device intended for use inside the body of the patient. In the implantable embodiment of the Bodell device, a tubular casing serves as a shunt either from the pulmonary artery to the left atrium of the heart (FIG. 6), or more generally between an artery and a vein (FIG. 10). A multitude of parallel-connected capillary tubes are used to oxygenate and/or purify the blood circulating through the casing.
FIGS. 3-5 of the Mortensen patent show a transvenous oxygenator made of a plurality of small diameter gas-permeable tubes 32 connected to headers 34 and 36 at each end. However, the specific device disclosed by Mortensen has a significant disadvantage in that two incisions are required. The insertion process is also rather complex.
Taheri discloses a transvenous oxygenator having a single membrane 16 through which oxygen diffuses. The membrane is disposed within a sheath 18 and both are supported by a flexible wire 20.
Berry, et al., disclose an in vivo extrapulmonary blood gas exchange device having a bundle of elongated gas permeable tubes 12 bound at each end and enclosed within a respective air-tight proximal and distal chambers 28 and 30. A dual lumen tube is situated relative to the gas-permeable tubes such that an outer lumen terminates within the proximal chamber 28 and an inner lumen terminates within the distal chamber 30.
The Hattler patents disclose several embodiments of percutaneous oxygenators. In the simplest embodiment ('689), oxygen is circulated through a plurality of hollow, gas-permeable fibers forming loops inserted through a single incision into a blood vessel. In other embodiments ('809), the fiber loops are bisected and placed in fluid communication with a mixing chamber within a tip at the distal end of the device.
Tanishita, et al., disclose an extracorporeal oxygenator (FIGS. 1A and 1B) in which diffusion of gases was enhanced by application of pulsatile flow superimposed on a steady mean flow. Flow pulsation is introduced in the oxygenator chamber by directly vibrating its bottom plate.
3. Solution to the Problem
The problem of streaming appears not to have been recognized in prior art percutaneous oxygenators. None of the prior art references known to applicant shows a percutaneous oxygenator that can be inflated and deflated to minimize streaming, and thereby maximize cross-diffusion of gases between the patient's blood stream and the oxygenator.